Skeleton of an australopithecine boy, on display at the Iziko South African Museum in Cape Town.
Photograph: Mike Hutchings/Reuters

Australopithecus sediba, the small hominid found in a cave in South Africa in 2008, and hailed as a possible ancestor for Homo sapiens, may not have been a fan of the crunchiest high fibre diet.

Palaeoanthropologists had conjectured that, like other potential human ancestors, the creature chewed bark and cracked nuts and other plant products.

But new research in the journal Nature Communications suggests that one of humankind’s most recently identified predecessors had a bark much worse than its bite. Had it really tried to live on nuts, it might have dislocated its own jaw.

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“Most australopiths had amazing adaptations in their jaws, teeth and faces that allowed them to process foods that were difficult to chew or crack open. Among other things, they were able efficiently to bite down on foods with very high forces,” said David Strait, an anthropologist from Washington University in St Louis, in the US.

But, said his colleague Justin Ledogar of the University of New England in Australia, “Australopithecus sediba is thought by some researchers to lie near the ancestry of Homo, the group to which our species belongs, yet we find that A. sediba had an important limitation on its ability to bite powerfully; if it had bitten as hard as possible on its molar teeth using the full force of its chewing muscles, it would have dislocated its jaw.”

The fossilized skeleton was found in the Malapa Cave 40 kms west of Johannesburg and, unusually, was found in nearly complete form. Researchers had enough information to make a computer-based model of the skull, to examine the wear on its teeth, and to test its table manners using much the same technology that tests the durability of car and aeroplane parts.

And the conclusion was simple: A. sediba had the ability to walk upright on two legs but it simply did not have what it takes to subsist on a diet of hard foods. Paradoxically, that doesn’t mean it could not have been a potential human ancestor. Human evolution may have been shaped by dietary changes and the search for varied food.

“Humans also have this limitation on biting forcefully and we suspect that early Homo had it as well, yet the other australopiths we have examined are not nearly as limited in this regard. This means that whereas some australopith populations were evolving adaptations to maximise their ability to bite powerfully, others, including A. sediba, were evolving in the opposite direction,” said Dr Ledogar. And his colleague Professor Strait said “Some of these ultimately gave rise to Homo.”